1. Field of the Invention
This invention relates to an underfloor structure for an automobile.
2. Description of the Related Art
Conventionally, there is known an underfloor structure for an automobile, the aerodynamics of which can be improved by flattening the underfloor. FIG.1 shows a plan view of the above-referenced underfloor structure. In the figure, reference numeral 1 designates a vehicle body of the automobile and 3 a center floor. A floor tunnel 4 is arranged at intermediate of the center floor 3 to extend from a front side of the vehicle body 1 through a rear side thereof. Along a direction extending from the floor tunnel 4 through a rear floor 6, a transmission 5, a drive shaft 8 and a rear differential gear 7 are arranged in order. In order to smooth an underside of an engine room 10 as possible, a flat undercover 113 closes a lower opening of the engine room 10 which is defined by a left front wheel housing 9, a right front wheel housing 9 and a dash lower panel 11, whereby a velocity of air flow flowing underfloor is increased to reduce the resistance of the air and the lifting force.
When the lower opening of the engine room 10 is closed by the undercover 113, the discharged heated air passing through a radiator (not shown) may stay in the engine room 10, and then the temperature and pressure in the engine room 10 will tend to rise. Consequently, most of the known undercovers 113 have a large number of louvers 113a for discharging the heated air staying in the engine room 10. This underfloor structure is disclosed in Japanese Utility Model Laid Open (Kokai) No.60-10552, for example.
In the conventional structure mentioned above, however, since an air flow of a low flow rate, i.e., a slow stream (current) is discharged from almost all area of the undercover 113 through the louvers 113a, an air flow of a high flow rate, i.e., a rapid stream flowing from a front of the vehicle body 1 is decelerated under the undercover 113 by the slow stream discharged through the louvers 113a. Therefore, a velocity distribution of the streams flowing under the center floor 3 and the rear floor 6 is as shown with U11, U12, U13 and U14 of FIG.1 in which a length of each arrow therein corresponds to the velocity of each stream. The distribution suggests that the velocity is lowered at the center of the floor in a direction of width of the vehicle.
According to our wind tunnel experiment under a vehicle speed of 120 km/hour, it has been found that the velocity of the cooling wind is relatively low around the transmission 5 and the rear differential gear 7 both of which constitute so-called "heat-radiant parts" existing under the floor, whereby they cannot be cooled down sufficiently. On the contrary, since the rapid stream, which is flowing under side floors arranged on both sides of the vehicle, strikes against rear wheels 15 directly, the aerial resistance is increased around the rear wheels 15. Furthermore, such a distributuion of velocity involves a phenomenon where a lifting force acting on the rear wheels 15 is larger than that acting on the front wheels 19, so that the driving stability would be influenced because of its inequilibration.
Further, in case of traffic retardation, since the heated air discharged from the engine room 10 through the louvers 113a is apt to stagnate around of the undercover 113 without flowing backward, there is a possibility that the heated air discharged from the engine room 10 may be sucked into the radiator again.
Under the above-mentioned circumstances, in order to reduce the above-mentioned aerial resistance against the wheels and to increase the cooling effect of the air flow on the heat-radiant parts existing under the floor, we have already proposed an underfloor structure consisting of an undercover 213 as shown in FIG. 2 in Japanese Patent Application Serial No.4-138440 (not published).
In order to control one air flow discharged from a front high-pressure area of the vehicle and the other air flow flowing under the undercover 213, it includes a pair of ducts 217 arranged at a rear part thereof and a narrow part 221 arranged between the ducts 217 to narrow the latter air flow.
In operation, the former air flow through the ducts 217 is decelerated and divided into two slow streams U3 flowing to the rear wheels 15, respectively, as shown in FIGS. 3 and 4. Therefore, the aerial resistance acting on the rear wheels 15 can be reduced in comparison with the resistance in case of rapid streams. On the other hand, the latter air flow flowing from the narrow part 221 along a center line of the vehicle is accelerated by its throttle effect and changed to a rapid stream U2. Consequently, due to this speeded-up air flow, it is possible to improve the cooling effect of the air flow on the heat-radiant parts positioned under the floor of the vehicle.
However, the above-mentioned underfloor structure still contains some problems to be solved.
First, as indicated by arrows in FIG. 3, the air flow from the vehicle front is throttled by clearances between the respective slow streams U3 discharged from the ducts 217 and the front wheels 19. Consequently, the air flow passing through the clearances is so accelerated that areas C of a high velocity are formed behind the front wheels 19, whereby the aerial resistance is increased due to the uneveness of the vehicle. Further, since this air flow, which is usable for cooling front brake units primarily, escapes sidewardly of the vehicle without striking the front brake units, the cooling operation thereon cannot be effected sufficiently.
Second, in the area B where the rapid stream U2 narrowed by the narrow part 221 is flowing, an air pressure (a negative pressure) is raised or recovered by a diffusion of the slow streams U3 in the areas A beside the area B. Therefore, the rapid stream U2 cannot be produced in the neighborhood of the rear differential gear 7 as one of the heat-radiant parts, so that it cannot be cooled effectively.
Third, since the rapid stream U2 after passing through the narrow part 221 flows apart from the transmission 5 relatively as shown in FIG.4, the air flow for cooling side surfaces of the transmission 5, which is also one of the heat-radiant parts, cannot be ensured sufficiently.